The present invention relates to a switching power supply, particularly to a switching power supply suitable for driving a load whose load current may fluctuate abruptly.
The DC/DC converter is a well-known switching power supply. The typical DC/DC converter uses a switching circuit to once convert a direct current input to an alternating current, uses a transformer to transform (step up or step down) the voltage of the alternating current, and uses an output circuit to convert the transformed alternating current to direct current. The DC/DC converter thus provides a DC output of a different voltage from the input voltage.
The switching power supply of this type is equipped with a control circuit that detects the output voltage and controls the switching operation of the switching circuit based on the detected value. This enables the switching power supply to supply a stable operating voltage to the driven load.
When the conventional switching power supply drives a load whose load current (output current from the viewpoint of the switching power supply) changes abruptly, however, it cannot easily maintain a stable output voltage.
This is particularly a problem when driving a CPU (central processing unit), a DSP (digital signal processor) or other such device that has a low operating voltage and requires a large amount of current in the active state but only a slight amount of current in the inactive state. In such a case, the output voltage of the conventional switching power supply is likely to change markedly with an abrupt change in the output current. As CPUs and DSPs operate at very high speeds, moreover, they are apt to malfunction when the output voltage changes, unless it is promptly stabilized.
It is therefore an object of the present invention to provide a switching power supply suitable for driving a load whose load current may fluctuate abruptly.
Another object of the present invention is to provide a switching power supply that reduces change in output voltage caused by abrupt change in output current.
Another object of the present invention is to provide a switching power supply equipped with a control circuit capable of quickly recovering from change in output voltage caused by abrupt change in output current.
The present invention achieves these objects by providing a switching power supply comprising: a main circuit unit including a switching circuit for converting a DC input voltage to an AC voltage and an output circuit for rectifying the AC voltage to produce a DC output voltage; and a control circuit for controlling the operation of the main circuit unit, the transfer function of the control circuit assuming a first value when the load current supplied by the main circuit unit changes at a rate not exceeding a prescribed rate and assuming a second value exceeding the first value when the load current changes at a rate exceeding the prescribed rate.
The present invention markedly improves transient response when the load current changes at a rate exceeding the prescribed rate because the transfer function of the control circuit is increased relative to that under normal condition.
In a preferred embodiment of the present invention, the second value is set at a level such that a transfer function of a closed loop composed of the switching circuit and the control circuit causes oscillation of the output voltage.
In another preferred embodiment of the present invention, the control circuit includes at least an amplifier that receives the output voltage or a voltage covariant therewith at an input thereof and the switching power supply further comprises means responsive to a change in the load current at a rate exceeding the prescribed rate for changing the level at the input of the amplifier.
In another preferred embodiment of the present invention, the aforesaid means is responsive to an increase in the load current at a rate exceeding the prescribed rate for changing the level at the input of the amplifier in one direction and is responsive to a decrease in the load current at a rate exceeding the prescribed rate for changing the level at the input of the amplifier in the opposite direction.
In another preferred embodiment of the present invention, the aforesaid means is responsive to a change in the load current at a first rate exceeding the prescribed rate for changing the level at the input of the amplifier to a first level and is responsive to a change in the load current at a second rate exceeding the first rate for changing the level at the input of the amplifier to a second level.
In another preferred embodiment of the present invention, the control circuit receives the output voltage or a voltage covariant therewith at an input thereof and includes at least a variable amplifier that assumes a first characteristic when the load current changes at a rate not exceeding the prescribed rate and assumes a second characteristic different from the first characteristic when the load current changes at a rate exceeding the prescribed rate.
In another preferred embodiment of the present invention, the first characteristic is a first gain and the second characteristic is a second gain exceeding the first gain.
In another preferred embodiment of the present invention, the control circuit includes at least a plurality of amplifiers that differ in characteristics and each receive the output voltage or a voltage covariant therewith at inputs thereof, a plurality of PWM (pulse-width modulation) control circuits associated one each with the amplifiers for producing control signal groups based on outputs of the associated amplifiers, and a selector for selecting one signal group from among the control signal groups based on whether or not the load current changed at a rate exceeding the prescribed rate, the operation of the main circuit unit being controlled based on the selected control signal group.
In another preferred embodiment of the present invention, the control circuit includes at least a plurality of amplifiers that differ in characteristics and each receive the output voltage or a voltage covariant therewith at inputs thereof, a selector for selecting one output from among outputs of the plurality of amplifiers based on whether or not the load current changed at a rate exceeding the prescribed rate, and a PWM control circuit for producing a control signal group based on the selected output, the operation of the main circuit unit being controlled based on the control signal group.